Installation of the Shell/Esso Brent B Condeep Production Platform

ABSTRACT A brief description of the soil and site conditions for Shell/Esso Brent B, Condeep Production Platform, is given. The instrumentation of the platform for the installation phase is outlined, and some results obtained during penetration of the base of the structure into the sea bed are presented. INTRODUCTION The first concrete gravity structure installed in the Northern North Sea was the Ekofisk Doris tank for Phillips Petroleum, placed in the summer of 1973. The next two to follow were the Condeep Production Platform Beryl A for Mobil and Brent B for Shell/Esso, both installed in the summer of 1975. The water depths at the three fields are 70, 118, and 140 m respectively. The greatest concern for installation of the Ekofisk tank was the seafloor topography and the load distribution on the bottom slab, as this structure was only equipped with 40 cm high concrete ribs. The seafloor at Ekofisk, however, proved to be almost perfectly flat and no problem related to an uneven seafloor arose. The Condeep structures are equipped with steel skirts. In order to prevent damage to the skirt during installation, steel pipe dowels extending below skirt tip level are used to keep the structure from moving laterally during the first phase of penetration known as "touch down". The object of the skirts is to improve the foundation stability and make the structure less vulnerable to erosion, and also for grouting purposes. The actual installation on the seafloor is a critical phase in the life of the platform and has to be included in the foundation design analysis. It should be checked that neither the structure nor the foundation soils are damaged or unfavourably affected during installation. This requires a comprehensive instrumentation and logging of all important factors, including, (1) platform inclination, (2) water pressure, in skirt compartments, (3) contact pressure on the base of the structure, and (4) strains within the structure. The Shell Brent B Condeep platform has been selected for an extended instrumentation program. This research project is sponsored by a number of oil companies and organizations both in Great Britain and in Norway. The Condeep Brent B structure was designed by A/S Hoyer-Ellefsen and constructed by Norwegian Contractors. The Norwegian Geotechnical Institute has acted as geotechnical consultant on the project, and has been responsible for the major part of the instrumentation. Dr. Olav Olsen is consultant on the concrete structure. The Aker Group designed and installed the ballast system. The Central Institute for Industrial and Scientific Research developed and produced the monitoring and data acquisition system. The River and Harbour Laboratory carried out special model tests for the installation phase, and measured the waves on the site. Bloms Oppmaaling A/S was responsible for the positioning, and Entreprenorservice A/S undertook the grouting. Det norske Veritas is the certifying society, and all the above mentioned organizations are based in Norway.